Somewhere a church bell started to chime, the first church bell Mary had heard in England for months. And then an air raid siren coughed and wailed.

Ernst sat in a crowd, all of them men of the Twenty-sixth Division of the Ninth Army, on the road above Boulogne's harbour wall. His pack was heavy on his back, and his rifle gleamed in his hands, polished until it shone. The men sat about, smoking gloomily, complaining about their officers, swapping stories about French women and wine, and tending to their feet – doing what soldiers always did. Ernst's Wehrmacht uniform was stiffly laundered, made smart for England. The men had dreaded these hours of waiting at their embarkation points, for they, and indeed the waiting fleet, were so obviously vulnerable to air attack. But there had been no sign of the RAF. Perhaps Goering had at last done what he promised, and beaten back Britain's planes for the day.

It was misty and cold. This was S-Day Minus One, the eve of Sea Lion Day itself. Ernst was looking out to sea. And before him an astounding spectacle unfolded.

Beyond the harbour the sea was crowded with ships. Heavy steamships glided in the deeper water, shadows on the sea, laden with stores and the vehicles of the motorised units. Smaller vessels plied the nearer waters, motor-boats and fishing smacks and even a few rowing boats. There were some exotic craft, such as the new varieties of assault boats like the one Josef had played with, and 'Herbert ferries', actually sections of pontoon bridges fitted with motors, stable and massive enough to carry over a complete anti-aircraft unit. All these specialised craft had been designed and built in the fever-pitch hurry of this invasion summer.

But it was the barges themselves that were the most remarkable sight. Many of them had already been towed out of the harbour, and they were forming up in great columns, convoys miles long. Black smoke rose in threads from the steamers that dragged them. There had been no rehearsal for this immense choreography of wood and iron and military force, for none had been possible.

And then yet another wave of planes went roaring overhead, sweeping out to sea: Messerschmitts and Junkers and Stuka bombers, ploughing determinedly towards England, to beat off the RAF and the Royal Navy, and to soften up the landing sites. A wave of Ju-52 transports followed, bearing paratroopers to begin the invasion from the air.

It was a magnificent spectacle, he told himself: a conjunction of forces, on land, at sea and in the air, the largest invasion across this ocean since the Romans. He would write a book about it one day. But for now Ernst felt very small, very vulnerable, a tiny disregarded piece of a vast machinery.

And somehow none of it seemed real. After the months of playful training, all the saloon-bar arguments about the relative strengths of navies and air forces and the sea-going capabilities of river barges, suddenly the order had come. It was strange to sit here and share a cigarette with a man, trying to believe that by this time tomorrow you might be in England, and there was a good chance that either he would be dead, or you would, or both, trying to believe this was serious, not just another exercise.

And here, out of nowhere, came Josef. He strode along the harbour wall, his black SS uniform standing out against the camouflage green battle dress. The men glared at him, or deliberately ignored him. Traditionalists in the Army had never accepted the SS. But Josef rose above it all. When he spotted his brother he beckoned.

Ernst glanced at his obergefreiter, who shrugged, his head wreathed in cigarette smoke. Ernst slipped his arms out of his pack, leaving it on the ground, and stood and crossed to Josef.

'Brother.' Josef shook his hand warmly. Then he studied Ernst's face. 'You don't seem very pleased to see me.'

'I'm pleased enough.' He glanced back at his unit. 'It's just, I don't know, I feel like a junior worker in a factory favoured by the manager.' In fact, that was the aura Josef gave off, with his strutting about in his glamorous uniform. But then, Ernst thought, 1940 was a good year to be a Nazi with ambition.

'Never mind these jealous dolts.' Josef said this loudly so the others could hear. 'Look, you should appreciate me being here. I've been pretty busy these last hours.'

'Doing what? Shagging that English girl?'

He laughed. 'No. Planning. Preparing. You must be aware of the detail involved in an operation like this. The Fuhrer's final commit order has been broken down in the planning until we are visualising every footfall of every soldier on every beach. As for Julia, don't mock her. She, and the rest of her Legion of St George, will be crossing in the second wave with me. I have a feeling Julia Fiveash is going to be very useful to us in the days of the occupation to come.'

'She's as mad as a rabid stoat.'

'You're much too cynical, Ernst. Look, I found you because Mother would want to know that we shook hands at least before we parted for England.'

Ernst was touched. 'Well, that's true. Thank you for finding me.'

'Not that that was easy, in a mob like this. Now listen to me, Gefreiter Ernst. You are caught up in the detail, you will be a mere pebble on those shingle beaches. But you must see the bigger picture. The Fuhrer has determined that Churchill will never be reasonable, that England must be eliminated from the war – and that we have just enough to make Sea Lion work. And so by the force of his personality he has brought his great generals together for the project. Even Goering!' He waved a hand. 'And now we are ready; you can see it. Goering has beaten back the RAF, just enough. The Kriegsmarine with its barriers of mines and purloined French ships can keep the Channel clear for the crossing, just enough. Even the weather is behaving itself – just! And so the Fuhrer has ordered that we go. Within six weeks we will have half a million men in England, the British army, weakened by Dunkirk, will be scattered, and Churchill will be suing for peace, if he has not been deposed or shot.'

Ernst said, 'Six weeks? In the ranks it is said that the Panzers will run out of petrol in three days.'

Josef snorted. 'I believe there is petrol in England.' He gripped his brother's shoulder. 'Listen to me. We will find each other tomorrow or the next day, the two of us, brothers on English soil. Yes?'

The obergefreiter nudged Ernst. 'Hey, Trojan. Smile for the camera.'

A truck was driving along the length of the sea wall, with a camera crew set up in the back, and a woman shouting directions. It was Leni Riefenstahl, who had followed the Nazis from Nuremberg to Poland, and now to the edge of the sea. The men waved and shouted cheerful obscenities.

More planes thundered overhead, in layers stacked up tall in the air, so many of them that they turned the grey afternoon sky black.